Aims. Recently, substantial flaring in the 6.7 GHz methanol maser line has been observed toward the high-mass young stellar object (YSO) S255 NIRS 3, where an accretion burst was also detected in the IR. Our goal is to study the change in the properties of the 6.7 GHz masers between the pre- and outburst phases, and investigate the connection between the maser and the accretion burst.

Methods. With the Karl G. Jansky Very Large Array (JVLA) and the European VLBI Network (EVN), we performed observations of the 6.7 GHz masers (covering a range in angular resolution from a few milliarcseconds to ≈1′′) during the burst phase and compared these observations with pre-burst measurements at similar spatial scales.

Results. The accretion burst and the subsequent increase in IR luminosity are very likely the origin of the 6.7 GHz maser flare. Since most maser centers operate in the unsaturated regime, a change by a relatively small factor (≈5) in the flux of pumping photons has produced an exponential growth in the maser intensity. The main pre-burst maser cluster is no longer detected during the burst. Compared to the pre-burst phase, flaring 6.7 GHz masers emit across a different VLSR range that is more strongly redshifted, and the emission extends over a larger area at larger separation from the high-mass YSO. In particular, the outburst peak emission originates from a remarkably extended (0.̋2–0.̋3) maser plateau at a radial distance of 500–1000 AU from the source.

Conclusions. Both the maser flare and the extraordinarily large extent of the maser structure can be a natural consequence of the burst in the accretion luminosity of the high-mass YSO. Our results strongly support models that predict IR radiative pumping for the 6.7 GHz CH3OH masers.

1. Introduction

The bright 51−60 A+ CH3OH maser transition at 6668.5 MHz has been recognized for a long time to be a selective signpost for recently formed luminous young stars (Lbol> 5 × 103L⊙). Its excitation requires radiative pumping by 20–30 μm photons through the second torsionally excited state (Sobolev et al. 1997). Intense IR radiation typically permeates the warm dusty environment surrounding massive young stellar objects (YSO). Although the emission of most 6.7 GHz maser sources is stable on timescales of (at least) several years (Sanna et al. 2010; Moscadelli et al. 2011), in some cases, strong flares of up to several 100 Jy have been recorded. In addition, in some sources, selected maser components show a well-defined periodicity (e.g., Goedhart et al. 2004). These variations can be naturally related to an occasional and/or periodic change of the background radiation (amplified by the maser) or the IR pump field.

Recently, we have reported on the first ever detected accretion burst from a massive young star (NIRS 3) in the star-forming region S255 (Caratti O Garatti et al. 2016). This detection was triggered by the serendipitous discovery of a CH3OH maser flare toward S255 NIRS 3 by Fujisawa et al. (2015) in 2015 November. Subsequent subarcsecond near-IR (NIR) observations showed that the K- and H-band fluxes of this source had increased with respect to the pre-burst level by 2.9 and 3.5 mag, respectively, suggesting a relationship with the maser flare (Stecklum et al. 2016). Additional IR observations proved that the integrated luminosity from NIR to millimeter wavelengths had grown from 2.9 × 104 to 1.6 × 105L⊙ (Caratti O Garatti et al. 2016).

This letter reports on observations made with the European VLBI Network (EVN) and the Jansky Very Large Array (JVLA) of the 6.7 GHz CH3OH maser emission in S255 NIRS 3 (hereafter NIRS 3) at the time of the outburst. We compare the new observations with previous interferometric and single-dish data, and discuss the change in the maser spatial distribution, structure, and flux.

2. Observations and data analysis

2.1. EVN 6.7 GHz CH3OH maser

We observed the 6.7 GHz CH3OH maser emission toward NIRS 3 with the EVN1 as a Target of Opportunity program on 2016 April 12 (code: RS002). We also reduced archival EVN observations obtained before the flare on 2004 November 6 (code: EL032). Both observations were conducted in phase-referencing mode by fast switching between the maser target (at a Doppler velocity of 5 km s-1) and a strong (C-band flux >0.1 Jy) reference position calibrator, J0613+1708 (for exp. EL032) and J0603+1742 (for exp. RS002). Left and right circular polarizations were observed with two (EL032) and eight (RS002) baseband converters (BBC), each BBC being 2 MHz wide. The EL032 and RS002 experiments were processed at the correlator of the Joint Institute for VLBI in Europe (JIVE) using an averaging time of 0.5 s and 2 s, respectively. Data were analyzed with the NRAO2 Astronomical Image Processing System (AIPS) following the VLBI spectral line procedures. Absolute positions of the CH3OH maser spots are registered with an accuracy of about ± 1 mas at each epoch. Additional information on the EVN observations is summarized in Table 1.